The principles of the invention will be more readily appreciated if one first considers the derivation of the annular kiln from the ordinary vertical kiln or shaft furnace.
With most forms of vertical kiln or shaft furnace for the treatment of, for instance, ferrous or non-ferrous ores, cement clinker, dolomite, lime or the like, the removal of the finished or heat-treated product from the lower, i.e. discharge, end of the shaft is rendered difficult by the fact that the whole mass of material under treatment filling the shaft should be encouraged to bear directly on the discharge means, so that the material will be engaged over the whole cross-section and the charge will sink as uniformly as possible through the shaft, thereby to maintain reasonably consistent gas permeability over the full effective cross-section.
Such difficulties were discussed for instance by H. Eigen (Zement-Kalk-Gips, October 1956, pages 454 - 456), K. Beckenbach (British Patent Specification No. 934,230), H. R. Suter (U.S. Pat. No. 2,861,788; assigned to L. von Roll, A.G.,) E. Spohn (Tagungsberichte der Zement-industrie, 1955, 11, 39 - 55), F. P. Somogyi, "The Vertical Kiln" (Cement Lime and Gravel, May 1952), and B.A.S.F (British Patent Specification No. 950,576).
These references also show that because of the lack of uniformity of the load across the shaft cross-section and the correspondingly uneven discharge rates from the centre to the perimeter or outer wall, not only is the material heat treatment irregular but it is also impossible, for example in the case of the calcination of limestone or dolomite, to obtain a product that is of a uniformly porous quality and in which an original size grading is retained as far as possible. As stated by K. Beckenbach, these characteristics can only be incompletely achieved even with the best of existing discharge devices and his approach was intended to discharge the material from the shaft furnace uniformly and without undue crushing, by the introduction of a fixed discharge table which, when disposed below the lower opening of the furnace and having a larger diameter than that opening, introduced stabilising and load-shedding characteristics according to the angle of repose of the material at the perimeter, thus leaving the extractor gear free to complete the feeding off action.
Furthermore the uniform treatment of the material in a shaft system can be seriously interfered with by the loosening action which must take place at the perimeter or outer wall and which has hitherto limited the maximum diameter of most stationary shaft systems to a few metres.
Various attempts were made both to increase the capacity and to improve the performance of shaft kilns or preheaters by the inclusion of core sections, thereby introducing the annulus principle, but in every case these have again been based on static arrangements with no relative movement between the outer shaft and the inner core walls; an interesting attempt was also made by B.A.S.F. both to incline and to rotate a shaft kiln so as to prevent hold-ups in the burning zone and give a more intimate mixing of the solids and hot gases. The fact that all of these arrangements would prove extremely complicated in both their manufacture and operation appears to have hindered their practical application.
In U.S. Pat. No. 2,842,350, M. Berz proposed a fixed annular shaft furnace in which the central opening in the annular floor, for the discharge of solid material which had descended through the annular shaft, was smaller than the inner diameter of the annular shaft. A ring structure was supported coaxially above the centre floor opening, forming a channel for gas to be passed upwards and thence through the annular bed, while treated solid was discharged over the edge of the floor under the ring structure. The gas was to enter the bed of solid material through the surface of the solid charge material sloping down from the inside bottom of the annular shaft, to the top of the ring structure.
Our own approach to this general problem can be said to have started with the annular preheater designed by R. V. Beal and L. H. Bishop, (British Patent Specification 828,886) in which we managed to overcome the main disadvantages of the Berz annular preheater by putting an eccentric discharge port in the floor and by rotating the bowl constituted by the floor and the wall and adopting an external cam track to work the pusher discharge rams to push the treated solids to the edge of the port. These measures virtually eliminated both the feeding and sealing problems which, at the fairly high operating suctions associated with deep bed up-draught operation, had been responsible for the lack of commercial success with the Berz design. The Beal approach had, however, still not fulfilled the essential need for continuous and uniform feeding of a granular or nodulised solid because, as will be more clearly explained later in this description, the bowl, its floor and the inner dome remained in fixed geometrical relationship to one another. Moreover the intermittent pusher discharge action could not help but impose unacceptably high disruptive forces on the solid material being treated and was unable to provide the uniformly distributed feed-off action and the regimented downward flow of the solids so essential for efficient contra-flow heat exchange.
In the annular preheater or hiln, described in our British Patent Specification 828,888 and U.S. Pat. No. 2,945,687, G. G. J. Davis took a significant step forward towards overcoming the above mentioned drawbacks, in that the floor or hearth on the one hand, and the walls of the annulus -- sometimes referred to as the bowl (outer) and dome (inner) -- on the other hand, were mounted to rotate about their respective central vertical axes, but these parallel axes were spaced from each other by a certain offset distance. The resulting precession and interaction between the bowl wall and the floor, which we shall call the Offset Feed Action, caused solids at the bottom of the bed to be pushed to and over the edge of the discharge port. The cam arrangements of the earlier versions were thereby dispensed with, and the effect of the offset action in this case was equivalent to an infinite number of radial pushers around the circumference, resulting in a solids flow of improved smoothness under favourable conditions.
During the rotation of the Davis preheater through one complete revolution, the relative motion between the annulus and the floor is circular, i.e. the locus of a point in the rotating annulus with respect to the rotating floor is a circle having a radius equal to the offset distance, assuming that the annulus and floor are rotated in unison, i.e. at the same angular velocity and neither lags behind the other. Such rotation in unison would be obtained in practice only by applying suitable positive drive both to the floor and to the annulus; if the floor alone were positively driven, the reactions between floor, solid charge material and annulus walls would not cause the annulus to rotate without some slippage with respect to the floor, so that the above mentioned locus would depart from circular and tend towards a cycloid curve.
According to U.S. Pat. No. 3,403,895, assigned to Dravo Corporation, P. E. Hatfield and L. H. Jacquay attempted to emulate the relative circular motion between annulus and floor of the Davis preheater, without actually revolving the annulus or the floor about their own central vertical axes, by imparting an orbital motion to either the floor or the annulus only, e.g. by rotating the vertical axis of the floor about the vertical axis of the annulus. Their ostensible purpose was to reduce the power requirement of the system; it would, however, require more power than the Davis system and it could only handle a solid charge capable of withstanding prodigious crushing forces, largely because it could permit no tangential slip between floor, charge and annulus walls.
The advantage of providing some tangential movement, in a partially analogous situation, was noted by C. Candlot in U.S. Pat. No. 1,429,925. Candlot mounted a horizontal grate below a shaft furnace, leaving a peripheral space between the bottom of the shaft and the grate below it. The grate was subjected to a planetary movement whereby solid charge material from the furnace, resting on the grate, was withdrawn radially outwards and spilled over the edge of the grate to a hopper. According to Candlot the planetary movement caused a tangential movement of the material covering the grate, thus avoiding any jamming that night otherwise occur between the grate and the shaft.
It is important to appreciate that all the annular preheaters and kilns thus far referred to, have relied for their discharge substantially entirely upon a pushing action applied by a circumferential member such as a ram or a wall, to compel the charge material to move inwardly from the bottom of the annular bed proper, across the floor to the discharge outlet in the floor. This pushing or ramming action, as will be more fully explained later, involves the exertion of considerable forces upon the actual charge material and upon the bearing supports of the equipment, with concomitantly high rates of degradation of the charge and wear on the machine parts.
Our British Patent Specification 1,059,149 and U.S. Pat. No. 3,331,595, both describe some modifications made by P. H. Nelson to the Davis annular preheater or kiln, mentioned above, having the offset feed action. These modifications included (1) a change from the flat horizontal floor, to a floor sloping conically downwards towards the centre at a shallow angle to the horizontal, and (2) the provision of sealing means at the extremities of the bowl and dome which permit quite independent rotation of the three components: hearth or floor, bowl or outer wall, and dome or inner wall.
The Nelson-Davis annular heat exchanger which forms the subject of British Patent Specification 1,059,149 and U.S. Pat. No. 3,331,595, operates like its predecessors in pushing the charge material across the hearth for discharge through the aperture, except under particular circumstances to which the present invention is directed, and which are applicable by virtue of the two modifications mentioned above. We have found that apparatus of the immediately foregoing type providing an annular bed of descending granular or nodular material, can be operated in a decisively advantageous manner if the geometrical design and control of the apparatus bear a suitable relationship to the physical properties of the charge material of the bed. When the relationship is suitable, the apparatus may be said to operate in a critical mode, in which the pushing action already referred to is superseded by action of a different kind, and advantages enumerated in the following description are attained.
It is an object of the present invention to provide conditions for operating an annular bed, of descending granular or nodular material, in the critical mode referred to.